In order to meet various ANSI requirements for digital data communications, telephone subscriber copper wire lines must meet specified industry standard performance criteria, which limit the operational range of a two-wire loop. For example, in the case of currently installed ISDN basic rate digital subscriber lines (having a data rate of 160 kilobits per second, including bidirectional data payload and overhead maintenance channels), the ANSI standard T1.601 for 2B1Q (two-binary/one, quaternary/four level) modulation, two-wire, full-duplex data transport with echo cancellation, typically describes an ISDN channel as one that does not exceed a two-wire loop loss of 42 dB at 40 KHz, or 1300 ohms, resistive. As a consequence, the operational range of such a two-wire loop is limited to a range on the order of 15-18 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN transceiver interface equipment.
To extend ISDN communications to the approximately twenty percent customer premises market that lies geographically beyond this range, it is necessary that the service provider either install repeaters in the loop, or use a different communication medium, such as a T1 carrier fiber optic link. Unfortunately, each of these alternative solutions to the extended range problem carries with it a substantial cost penalty that the customer is unwilling to bear.
For example, the repeater approach requires the installation of both an office end repeater powering unit, plus a repeater mounting pole, or a subterranean, environmentally hardened housing (bell jar) for the repeater. Not only does this involve the use of additional equipment (including the cost of the repeater hardware and its installation), but it entails the expense and labor of maintaining the repeater enclosure.
Similarly, although T1 channel banks, located in both the switch office and another downstream location (office or remote hut) that is geographically `close` to the subscriber premises, are able to accept basic rate interface transmission extender (BRITE) cards for T1 carrier extension, the fact that T1 carrier systems are configured to include capacity for multiple extended basic rate services means that their use to deliver only a single basic rate extended service is prohibitively expensive and impractical.